In recent year, a solar cell (solar cell module) has been widely employed as a device directly converting solar energy into electric energy from the viewpoints of effective use of natural resources and ecology. Further development of solar cells is in progress.
A solar cell generally has a structure that photovoltaic elements are sealed through a light-receiving side sealing film and a backside sealing film between a transparent light-receiving side protection material and a backside protection material (backside covering member). In order to generate a large electrical output, a conventional solar cell is employed in the form of a solar cell module having plural photovoltaic elements connected to each other. Therefore, sealing films having high insulation properties are employed for ensuring an electrical insulation property.
In the solar cell, it is strongly required to take in incident light into photovoltaic elements as effectively as possible in term of improvement of generating efficiency. Therefore, it is desired that a light-receiving side sealing film has transparency as high as possible and does not absorb and reflect incident solar light so that it transmits most of the incident solar light.
In the light of transparency, films made of ethylene-unsaturated ester copolymer such as ethylene-vinyl acetate copolymer (EVA) are used as solar cell sealing films. Furthermore, addition of crosslinkers such as organic peroxides to the ethylene-unsaturated ester copolymer for improving crosslink density brings about improvements of strength and durability of the sealing film.
Moreover, Patent Document 1 shows that addition of a compound containing alkyleneoxy group such as tetraethylene glycol di(meth)acrylate brings about an improvement of transparency of the sealing film.
A solar cell is, for example, prepared by superposing a transparent light-receiving side protection material, a light-receiving side sealing film, plural photovoltaic elements, a backside sealing film and a backside protection material (backside covering member) in this order to give a laminate, and subsequently pressing the laminate under heating to crosslink or cure a light-receiving side sealing film and a backside sealing film. Thus, the laminate is adhesively combined by crosslinking or curing a light-receiving side sealing film and a backside sealing film.